The present invention relates to a sealed rolling bearing which has its bearing space sealed by seals.
Among sealed rolling bearings which have their bearing space sealed by seals to avoid leakage of a lubricating agent such as grease filled therein, there is a type which has an outer ring, an inner ring, rolling elements disposed between the inner and outer rings, and seal members secured to one of the inner and outer rings, the seal members each having seal lips kept in sliding contact with the opposing metal face of the other of the inner and outer rings or the surface of a metallic member mounted to the other of the inner and outer rings, such as a slinger.
II. Description of the Related Art
In order to ensure good sealing performance with this type of sealed rolling bearings, it is required that the seal lips, which are made of rubber or the like, be brought into sliding contact with the metal surface with a sufficiently high contact pressure. In order to ensure high sealing performance, Japanese patent publication 55-63010 proposes to form each of the seal members so as to have a plurality of seal lips including an inner lip, an outer lip and a middle lip having forked tips to increase the number of seal points where the seal lips slidably contact the metal surface.
In recent years, requirements for energy saving including fuel consumption saving are becoming more and more strong for various devices such as automotive wheel bearing assemblies using a sealed rolling bearing. In order to meet these requirements, it is necessary to minimize the torque loss of bearings while keeping sufficient sealing performance. However, increasing the contact pressure of each seal lip or increasing the number of seal lips to insure the sealing performance will result in increased friction torque of the seal lips attendant to sliding contact with the metal surface, which will in turn increase the torque loss of the bearing.
As one solution to this problem, Japanese patent publication 2003-262231 proposes to subject the metal surface with which the seal lips slidably contact to hardening or form a corrosion-resistant or rust-resistant hard film on the metal surface to improve the abrasion and corrosion resistances at the metal surface and decrease the wear of the metal surface, thereby ensuring the sealing performance while decreasing the initial interference of the seal lips, thereby decreasing the friction torque attendant to the sliding contact of the seal lips on the metal surface. With this arrangement, according to this publication, it is possible to reduce the wear of the metal surface with which the seal lips are brought into sliding contact even in a sealed rolling bearing used in environments where it is exposed to muddy water or aqueous solutions containing salt such as snow melting agents, such as a wheel bearing assembly of a motor vehicle used in cold regions.
The solution proposed by JP patent publication 2003-262231, in which the metal surface is hardened or formed with a hard film, can hardly reduce the initial interference of the seal lips. This is because even before the publication 2003-262231, it was well-known to harden bearing rings or slingers with which the seal lips are brought into sliding contact. Thus, these older bearing rings or slingers were already sufficiently hard and thus sufficiently wear-resistant. Specifically, among hardening methods recommended in the publication 2003-262231, hard chrome plating gives surface hardness of Hv 800 to 900, and non-electrolysis nickel plating gives surface hardness of Hv 500 to 800. These values are not so different from the surface hardness of older raceway rings.
Also, the friction coefficient between seal lips, which are typically made of rubber, and metal does not have any relation to the surface hardness of the metal. Therefore, the method of the publication 2003-262231 does not effectively decrease the friction torque attendant to sliding contact of the seal lips on the metal surface.
An object of the present invention is to provide a sealed rolling bearing which can decrease the torque loss while ensuring the sealing performance.